Since the late 1980s a great deal of research around the world has focused on developing methods to incorporate fully exfoliated smectite clays into polymers to increase mechanical and barrier properties. The typical approach relies on organoclay technology developed by Jordan in the 1950s, wherein the clay surface is treated to render it compatible with hydrophobic materials like the polyolefins and waxes. This surface treatment consists of an adsorbed monolayer of a high-molecular-weight quaternary amine, such as dimethyl dihydrogenated tallow amine. The surfactant adsorption takes place via an ion-exchange reaction involving the negatively charged basal surface of the clay platelets.
The simple mechanism by which the organoclays can improve barrier properties relies on the high aspect ratio of the exfoliated clay platelets to impart a tortuous path that retards the transport of diffusing species like oxygen or water vapor. In a strictly tortuous path mechanism, all diffusing species would be retarded to the same degree. The tortuousity factor can be as high as several-hundred-fold for impermeable platelets with aspect ratios of 100–500 and at modest mineral loadings of 5–10 volume percent. Unfortunately, nanocomposite performance has not always lived up to expectations, and barrier improvements of two- to four-fold or less are more typical.
To overcome the difficulties in exfoliating organoclays in hydrophobic polymers like the polyolefins, researchers have used functionalized polymers, like maleated polyethylene and polypropylene, as dispersants. While polar functional groups can interact with the organoclay surface and compatibalizing agents can promote exfoliation, this approach to nanocomposite formation has provided only modest improvements in the mechanical properties of polyolefins. Moreover, there have not been any published results that show increased barrier toward oxygen or water vapor in polyolefins or waxes.
Accordingly, there is a need for a rational approach to the design of new organoclay chemistries that provide both melt- and solid-state miscibility and enable the preparation of nanocomposites demonstrating significant improvements in the control of polymer nucleation, crystal growth, and physical properties such as increased mechanical and barrier performance.